This a continuation of International Application No. PCT/US94/01228 filed Apr. 18, 1994.
1. Field of the Invention
This invention relates to vehicle suspensions, and more particularly, to trailing arm suspensions with axle lifts.
2. Description of Related Art
Several types of vehicles, including semi-trailer truck vehicles and the like, have multiple sets of axle/wheel assemblies arranged to adequately support relatively heavy loads. To adjust the load support provided by these assemblies, it is known to employ suspension systems utilizing trailing arms pivotally mounted to the vehicle frame in combination with adjustably pressurized air springs and the like.
When the vehicle is carrying a relatively light load, it is desirable to relieve the load transmitting relationship between the vehicle in one or more of the axle/wheel assemblies, and also to disengage the axle/wheel assembly from ground contact to reduce tire wear. To relieve load support when an air suspension system is employed, air pressure can be reduced. To achieve disengagement of the tires from the road surface, devices commonly referred to as axle lift mechanisms can be employed. Prior lift mechanisms utilized stressed mechanical springs acting directly between a vehicle frame and the axle. When the downward load forces exerted on the axle by the suspension system were relieved, such as through deflation of air springs, lifting forces exerted by the mechanical springs pulled the axle upwardly to a raised position. These lift mechanisms required a sufficient spring stress to support the axle and the various suspension components in a raised position and substantially increased the spring stress when the axle was lowered, which undesirably imparted pre-load forces on the suspension system, reducing the actual maximum vehicle payload carried by the suspension system.
Improved axle lift mechanisms were later developed that overcame the problem of substantial payload reduction. These mechanisms generally comprised a lever arm or cam pivotally mounted to the vehicle frame, with the lever being connected to the axle/wheel assembly by a flexible member such as a strap or a chain and to a compressed or tensioned spring mounted to the axle. The lever arm or cam formed a moment arm with respect to the lever arm pivotal connection to the frame. The moment arm was minimized as the air spring was pressurized to move the axle/wheel assembly to the road engaging position. By minimizing the moment arm when the axle/wheel assemblies are in the road engaging position, the preload forces were minimized. Examples of these types of axle lift mechanisms are found in U.S. Pat. No. 3,771,812, issued Nov. 13, 1973 to Pierce et al. and U.S. Pat. No. 4,634,141, issued Jan. 6, 1987 to Hagan et al.
Axle lift mechanisms can also be used with trailers that are adapted for both highway and railroad use. The wheel gear upon which the trailers ride obviously depends upon whether or not the trailer is to be used in normal highway service or on a railroad track. In the former, conventional ground-engaging rubber tires are required, and in the latter, a rail wheel set, sometimes known as a rail bogie, must be used. Such mechanisms are not only retractable to lift an axle relative to a trailer, but are extendable to lift a trailer relative to the ground to permit attachment of a rail bogie.
Designs for an extendable, retractable suspension are disclosed in U.S. Pat. No. 5,058,916, issued Oct. 22, 1991 to Hicks, and U.S. Pat. No. 5,058,917, issued Oct. 22, 1991 to Richardson. Both patents disclose a retractable trailing arm suspension that is also extendable to lift the vehicle frame to provide for the insertion of a detachable rail bogie to the vehicle frame without the need for an additional lifting mechanism or device. Both structures are relatively complex and have a mechanical locking mechanism that locks the trailing arm of the suspension to the vehicle frame when the suspension is in the raised position. The complexity of these designs renders their manufacturing and operating costs relatively high.
The prior axle lift mechanisms and retractable suspensions satisfactorily performed their function. Unfortunately, their satisfactory performance was at the cost of undesirable complexity and increased maintenance. The prior axle lift mechanisms also required two pivotal connections; one for the trailing arm and one for the lever arm. The second pivotal connection increased the complexity of the lift axle by requiring separate mechanical connections between the spring and lever arm and the lever arm and trailing arm. It is desirable to have a lift mechanism that reduces the complexity and maintenance of the prior lift mechanisms, without sacrificing performance or durability.